Conventionally, in the electrophotographic image forming apparatus, a process cartridge type in which the photosensitive member and the process means actable on the photosensitive member are integrally assembled into a cartridge detachably mountable to the apparatus main assembly of the image forming apparatus has been employed. According to this process cartridge type, maintenance of the image forming apparatus can be carried out by an operator himself (herself) without relying on a service person, so that operativity can be remarkably improved. Therefore, this process cartridge type has been widely used in the image forming apparatus. The process cartridge includes a photosensitive drum (electrophotographic photosensitive drum) which is an image bearing member, a voltage applying device for imparting electric charges to the photosensitive drum, and other process means actable on the photosensitive drum. The process means generally include a developing means for supplying a developer (hereinafter referred to as “toner”) to the photosensitive drum, and a cleaning means for removing the toner remaining on a photosensitive drum surface without being transferred.
As a charging means in the voltage applying device, a roller charging means (type) using a charging roller has been widely used in recent years. In the roller charging type, the charging roller which is an electroconductive elastic roller is urged against and contacted to the photosensitive drum, and a voltage is applied to the charging roller, so that a surface of the photosensitive drum is electrically charged. The charging roller is in general in the form in which an entire longitudinal region of a core metal shaft except for both end portions is coated with an elastic layer, and as a supporting constitution, a method of rotatably supporting the core metal shaft end portions by supporting members as disclosed in Japanese Laid-Open Patent Application 2013-109209 is employed in general. Further, the supporting members are supported movably in a radial direction of the photosensitive drum by a frame of the process cartridge, and are urged toward the photosensitive drum by compression coil springs with a predetermined urging force (pressure), so that the charging roller is rotated by rotation of the photosensitive drum.
However, with speed-up and downsizing of the image forming apparatus in recent years, slip is liable to generate between the photosensitive drum and the charging roller. When the slip generates, a charge potential of the photosensitive drum causes a difference between a slip portion and a non-slip portion of the charging roller, so that stripe non-uniformity generates in an electrophotographic image in some cases.
As a countermeasure against the stripe non-uniformity, a sliding torque between the supporting member and the core metal shaft is lowered by reducing a diameter of the core metal shaft of the charging roller, whereby followability to the photosensitive drum can be enhanced. However, with a decreasing core metal shaft diameter, flexural rigidity of the charging roller lowers, with the result that there arises a problem such that the charging roller is not readily uniformly contacted to the photosensitive drum in an entire longitudinal region.